Clumping of Staphylococcus aureus in plasma has been suggested as a potential virulence factor..sup.1-5 Several mechanisms can be responsible for this aggregation. A fibronectin-binding protein has been suggested to cause aggregation of staphylococci in fibronectin at the concentration found in sera..sup.5,6 The presence of protein A causes staphylococci to aggregate in normal human sera, which frequently contain specific immunoglobulins directed against staphylococcal antigens..sup.7 Due to a high cell surface hydrophobicity, many staphylococcal strains auto-regulate under isotonic conditions..sup.8 It is believed that clumping of staphylococci in fibrinogen is caused by the so called clumping factor or fibrinogen-binding protein, situated on the staphylococcal cell surface..sup.1,9 Fibrinogen has also been suggested to mediate adhesion of S. aureus to cultured human endothelial cells.sup.10 and to catheters in vitro and in vivo..sup.11,12 It has been disputed whether clumping factor is distinct from coagulase.sup.1 or if it is a cell-bound form of coagulase..sup.13, 14 Staphylococcus aureus coagulases can be grouped into eight different serotypes.sup.15 and the existence of multiple molecular forms of coagulases has been suggested,.sup.16 although most investigators believe that lower molecular weight subspecies in coagulase preparations are due to proteolytic degradation of a larger protein..sup.17 Staphylococcal coagulases have been shown to induce polymerization of fibrinogen to fibrin by binding, and thereby activating, prothrombin. The coagulase-prothrombin complex causes the release of fibrinopeptides from fibrinogen in a manner similar to that described for thrombin in physiological blood clotting..sup.18 Fibrinogen precipitation and network formation can also be induced non-enzymatically, e.g. by exposing fibrinogen to various highly positively charged molecules like protamine, which interacts with specific negatively charged sites on the D-domain of fibrinogen..sup.19
We have recently described staphylococcal components that interact with fibrinogen and which can be purified from S. aureus culture supernatants..sup.13 These are an 87 kDa coagulase and a 19 kDa fibrinogen-binding protein. The 87 and 19 kDa fibrinogen-binding proteins are essentially extracellular proteins, but can to some extent be found on the staphylococcal cell surface. Thus, these proteins can give rise to the clumping phenomenon both by inducing coagulation and by direct fibrinogen-binding.
In this report we show that there are at least three distinct fibrinogen-binding proteins produced by S. aureus strain Newman, and that two of these proteins are coagulases.